Analyses of the hydrocarbon fluids from a reservoir are usually carried out by direct distillation of the sample, followed by gas chromatographic analysis of one or more distilled fractions. One major weakness of this technique, however, is that current techniques for this process typically do not produce extended compositional data (i.e., up to and including a C30+ fraction) that are necessary for use with modern compositional and process simulations. Instead, current techniques generally only provide data for the C12+ fraction, and possibly the C20+ fractions.
Hydrocarbon or petroleum reservoir fluids exist in formations as “live fluids.” Because the fluid is at elevated pressure and temperature conditions within the formation, non-hydrocarbon gases (e.g., N2, CO2, H2S) and light hydrocarbons (e.g., C1 to C5) are typically dissolved in the oil. During production, as fluid containing the hydrocarbons flows to the wellhead and subsequently to the surface separators, the gases and light hydrocarbons dissolved therein begin to evolve due to lower pressures and temperatures. The evolved gases thus can alter the composition of the original oil, as it existed in the formation, and two phases, a liquid phase and a vapor phase, emanate. A surface separator separates the liquid and vapor phases. Vapor is collected at the separator gas orifice and liquid at the separator oil outlet. These separate liquid and vapor streams are then analyzed in a laboratory to obtain their individual molecular compositions. The prior art techniques take the individual molecular compositions, and then mathematically recombine them using the original gas-oil ratio to produce a total well-stream composition, which typically includes the individual values of each of the hydrocarbon components up to a C12+ fraction. (That is, individual hydrocarbons are reported up to and including the C11 fraction, and a C12+ fraction is also reported). In a similar manner, a bottom-hole sample is flashed at ambient conditions to produce a flashed vapor, and the flashed vapor and remaining liquid are then each separately analyzed for their molecular compositions. Each of these compositions are mathematically recombined with the volumes of vapor and liquid recovered to produce the well stream composition, usually to C12+.
One prior art apparatus and method for compositional analyses utilizes a topping unit, a distillation column, and at least one gas chromatograph. The topping unit includes interconnected calibrated aluminum cylinders, with each cylinder being connected to a manometer with tubing and valves. The whole unit is enclosed with an insulated material and cabinet doors. The distillation system consists of a distillation flask, e.g. a 500 mL distillation flask, having a tapered joint, an inlet to charge the sample, and a thermometer to measure the still temperature. A heating mantle and a thermocouple are included for measuring the temperature of the fluid in the flask. It includes a low temperature distillation apparatus where the flashed oil is heated to about 608° F. and the vapor can be condensed in a condenser supplied with a circulation of cold water to leaving a liquid C12+ fraction residue. Evolved gas can be collected in the evacuated topping unit and the condensate cut fraction collected in a receiver. Both the gas and the condensate fractions may be analyzed by gas chromatography, and the density and molecular weight of the residual oil C12+ fraction measured and used to calculate the total fluid composition. This method suffers, however, in that it only provides compositional data up to the C12+ fraction.
Thus, there exists a need to provide methods of analyzing hydrocarbon fractions up to at least a C20+ fraction, preferably up to a C30+ fraction for use in advanced simulations.